Guys,
Dug out an old article that appeared in the October, '83 issue of Hot Rod
magazine. Title "400hp Ford 302W Buildup" written by Leonard Emanuelson.
Let me quote a few of the more pertinent statements in this article.
"Ford,on the other hand, has remained with the 'oversquare' philosophy of
large bore and short stroke, high-winding performance. So when comparing
the Fort to the Chevy, you must understand that no matter what you do, the
Ford will not match the Chevy's low end torque, so you have to build for
high rpm power. It's strictly a matter of physics. A Chevy with a 3.5-inch
stroke is going to make more torque than a Ford with a 3-inch stroke."
"Since this was the first Ford small-block I had dircted, I went back to
what Smokey (Yunick) had said: it doesn't know what name is on the outside.
Looking at the short-block you could say, "This is nothing more or less than
a 302 Chevy - it has a 4-inch bore, a 3-inch stroke, 90-degree Vee, and a
90-degree crankshaft." One shortcoming over the Chevy is the fact that it
only has four head bolts per cylinder to the Chevy's five, but being
normally aspirated and with low-compression ratios this isn't a factor. It
has a shorter length connecting rod than the 302 Chevy, which will help it
make better low and midrange torque. In fact, the rod-to-stroke ratio is
1.697 - a ratio similar to most big-block Chevys. The old 302 Chevy had a
rod-to-stroke ratio of 1.9, and while this seems like a bunch of meaningless
numbers it is one of the most important factors in designing an engine
combination. Efficient cylinder filling is what making good power is all
about, and a lower rod-to-stroke ratio motor has higher piston speeds (moves
the piston away from top dead center quicker) and can make use of a much
larger cylinder head port and a larger camshaft."
"Because of the rod-to-stroke ratio, we determined that we were going to
'over cam, over cylinder head, and over intake manifold' this engine.
Referring back to the rat motor, have you ever looked at the size of the
ports in a hi-po big-block Chevy? Also, past experience has shown us that
the more cam you use, the harder a rat will run. Well, what we have here is
a baby rat motor, and based on past experience, it was going to get plenty
of port volume and camshaft - regardless of what anyone else said."
Incidentally, the motor they built made 388 horsepower at 6500 and was still
climbing. It had an Edelbrock Torker manifold, 1-inch open carb spacer, 1&
3/4" Hedman "flat collector" headers, Holley 4776 600 cfm double-pumper
carb, 302 Clay Smith cam, and MSD-7A ignition. With an Edelbrock Performer
manifold, horsepower was down to 346 @ 6500, but was better than the Torker
below 5000 rpm.
This article was written back in '84. I think most recent articles on the
subject seem to give the edge to long rod-to-stroke motors with the same
kind arm waving about piston velocity, etc., etc.
Being a physicist, I tend to start pretty basic. When the spark plug fires,
about 1000 psi is generated over the piston. Now a given volume of gas,
let's say 72 cubic centmeters (which corresponds to 10:1 compression with a
302) at this pressure represents a certain amount of energy. When this gas
expands adiabatically, a certain fraction of the energy is transformed into
work. It's hard to see why changing the rod-to-stroke ration would change
the amount of work you can get out of the compressed gas, arguments about
torque notwithstanding. The same argument also holds for changing the
bore-to-stroke ratio for a given displacement. There may be some truth to
arguments about cylinder filling dynamics being affected by rod-to-stroke or
bore-to-stroke ratios. But simplistic arguments about torque, lever arms,
and thrust angles seems, under closer inspection, to be just more specious,
fuzzy thinking. In the above article, the result was essentially a
self-fullfilling prophesy. The motor was built to work best at high rpm and
it did, although this seems inconsistent with the statement that the short
Ford rods "make better low and midrange torque." Could we please see
dynamometer data for a comparable Chevy 302 with the 1.9 rod-to-stroke
ratio? Same cam grind, intake manifold, etc., etc.
Just some more food for thought.
Bob
|